Device isolation typically is achieved by utilizing local oxidation of silicon (“LOCOS”) or shallow trench isolation (“STI”) techniques. In the STI device isolation technique, isolation is typically achieved by forming a recess or trench in a layer that is destined to become two adjacent active areas, and filling the trench with an isolation material. The material in the trench, typically a nitride material, is referred to as a spacer. Nitride spacers, in addition to electrical isolation, may also be used as a fluid barrier.
STI is beneficial in providing higher packing density, improved isolation, and greater planarity, by avoiding the topographical irregularities encountered when using conventional thick film oxide isolation (LOCOS). In particular, the growth of thermal field oxide using a mask, such as nitride, creates an encroachment of the oxide into the active areas; this encroachment is referred to as the bird's beak effect.
Isolation using STI, however, has some limitations. For example, there is a relatively short diffusion path along the junction of the spacer and the underlying substrate for fluids (gas and liquids). Accordingly, there is an increased potential for leakage. Additionally, because the spacer is deposited on the surface of the substrate layer, the spacer is susceptible to shear forces which can lead to leakage and even failure of the device at the junction of the spacer and the underlying substrate.
What is needed therefor is a plug and method of forming a plug that overcomes one or more problems in known plugs. It would be beneficial if the plug and method of forming a plug could increase the diffusion path past the plug. It would be further beneficial if the plug and method of forming a plug could increase the strength of the plug-to-substrate layer interface.